Method for the complete separation of liquid air into oxygen and nitrogen



METHOD FOR THEbOMPLETE SEPARATION 0 LIQUID AIR INTO OXYGEN AND NITROGEN Filed Feb. 18. 1927 2 shiitl shqet 1 I A "MEs'sER 1,785,491

m wwww Dec. 16, 1930. A. MEssER IETHOD FOR THE COlPLETE SEPARATION OP LIQUID AIR INTO OXYGEN AND NITROGB" Filed Feb. 18, 1927 2 sbo ota sheot 2 3 Patented Dec. 16,1930} UNI-TED sTArEs PATENT. oFFlcE ,v ADQLF was, or FaANxroar-oN-rnE-mam, GERMANY.

METHOD Eon THE comrnmn snrnna'rion or 'Lmum am mro oxxem; AND NITROGEN Application filed February lg 1927, Serial No. 169,400, and in Germany February 18, 1926.

In the method of obtaining oxygen by the separation of air into oxygen and nitrogen, said oxygen is conveyed in a liquid or gaseous form from the collecting vessel of the separa- 5 tion apparatus directly-into a counter-current heat-exchanger wherein liquid oxygen is evaporated, and its cold given up to incoming compressed air.

If oxygen above the oxygen bath is withdrawn in the gaseous form, then it is impossible to obtain the same with a purity higher than 99% because the vapour is always richer in nitrogen than the liquid itself. Thus,

for example, the composition of the vapour above a liquid with a composition of 99% oxygen and 1% nitrogen is 97.37% oxygen and 2.63% nitrogen, or that of the vapour above a liquid with 99.7% oxygen and .3%

nitrogen is 99% oxygen and 1% nitrogen.

It is clear from this that where the oxygen is removed in gaseous form from the collecting vessel, it is impossible to produce oxygen of high per cent purity, that is oxygen of purity higher than 99% generally required in modern processes.

If, on the other hand, the oxygen is with-- drawn from the collecting vessel in the liquid form, then the above mentioned disadvantages do not appear since the oxygen with drawn has the same high percentage compo sition as the oxygen bath itself. The removal-of the liquid from said bath is attended, however, by the loss of the total heat of evaporation present in the oxygen withdrawn from said bath. To this must be added the losses which result from the radiation of cold in the insulation of the apparatus. The utilization of this heat of evaporation in the oxygen bath is, however, particularly important in separation apparatus with several a1r-separa-.

tion operations wherein nitrogen'under pressure is re-liquefied by means of liquid oxygen. Complete separation of the oxygen and nitrogen so that both gases are produced with a minimum purity of 99% can be' obtainedonly when suflicient nitrogenivapour under pressure is re-liquefled, since in this case only can sufiicient washing nitrogen be delivered to'the uppermost jected to suction.

expended without necessarily increasing t column which is sub As is known, the quantity of oxygen evaporated in practical operation in an oxygen 4 bath is not sufiicient for the complete liquefaction of suflicient washing nitrogen when the oxygen is withdrawn in the liquid form,

because the losses of cold to the outside are too eat. If, on the other hand, the oxygen is withdrawn in the gaseous form, a su'flicient quantity of nitrogen can be condensed because the quantity of. oxygen evaporating is no I then greater by about 20%, Apart from the disadvantage that the oxygen cannot be produced with a sufficiently column appa'ratusthat is, ap aratus with v double rectificationwherein t e oxygen is withdrawn continuously inthe gaseous con- .dition from the evaporator, particularly. I

when the apparatus is constructed as a pure nitrogen apparatus. Where, on the other hand, the oxygen, in two-column apparatus, leaves the evaporator inliquid form, explosions have never been observed, in spite of ractical. experience extending over a numer of years. This is clearly because scaveng ing is effected continuouslyv in the oxygen evaporator by the" removal, in liquid form, of the oxygen, sothat particles which might lead to explosive mixtures cannot settle." Hitherto to prevent explosions occurring nitrogen-producing plant wherein the oxygen is removed in gaseous form from the evaporator, large quantities-of liquid oxy en are withdrawn from the lowest part oft e oxy-. Y gen evaporator at intervals of a few hours the objectbeing to prevent the settling of f' explosive substances. This resultsin a not,

inconsiderable loss of cold and of energy immunity from explosions. The disadvantages mentioned above are obviated according to the invention by utilizing the entire heat of evaporation of the liqui oxygen for the recovery of 1i uid nitrogen by returning the former into t e warmer part of the apparatus (pressure stage) where the gaseous nitrogen is condensed with the aid of the cold liberated on the evaporation of the liquid oxygen.

For this purpose the oxygen is conducted from the lowest part of the evaporator into a coil in the part of the two-column apparatus, subject to pressure. Inside this coil the entire liquid oxygen is evaporated by the temperature difference (about 4 C.), a corresponding quantity of nitrogen vapour under pressure being simultaneously condensed on the outside of the coil.- In this manner the quantity of liquidwashing nitrogen is increased by about 20%, corresponding to the quantity of oxygen withdrawn, and a sufficient quantity of liquid washing nitrogen is obtained for the upper column subject to suction. With this arrangement the temperature. difference, which exists between the said parts of the air liquefying and separating plant is not fully utilized.

According to the invention full utilization is achieved by employing the liquid oxygen returned 'into the warmer part of the plant in such a manner that the oxygen leaves the plant finally at the temperature obtaining in the said part. This is effected, for example,

.by extending the coil, conveying the liquid oxygen in counter-current to the nitrogen vapour rising under pressure, to the vicinity of the pressure-reduction valve for the liquid air, and by causing the evaporated oxygen to emerge there. Whereas in the first explained method a temperature difference of about 4 C. only 182 to 178 0.), that is from to was utilized, it is, accord- "ing to the invention, possible to utilize the entire temperature difference together with the evaporation heat of the oxygen.

Figs. 1 and 2 of the drawings illustrate diagrammatically in longitudinal section two-column apparatus for carrying out the two methods.

In the apparatus illustrated in Fig. 1, precool'ed compressed air passes through a conduit a into the lower part of the separation apparatus, flows through a heating coil 0 in a bath 6 of liquid rich in oxygen and through a conduit d to.the pressure-reduction valve e, whence it finally emerges into a liquid condition, flowing through the lower colunm g in the lowest part of which a liquid rich in oxygen collects in the bath 1). This liquid rich in oxygen is maintained boiling by the coil 0 heated by the compressed air. The resulting vapours flow upwardly in the column 9 and serve for the washing of the oxygen containing liquid air flowing down therein. The vapours formed above the separating column 9 and flowing upwardlyconsist, depending on the pressure at which the column operates, mainly' of nitrogen and to a less degree of oxygen. The liquid rich in oxygen is conducted from the bath b through a contherethrough.

duit 2' into thesecond column m and is reduced to a lower pressure .by a valve is. The liquid oxygen thus obtains a lower boiling point so that nitrogen-oxygen vapours rising in the first column condense in the interior of condenser tubes 0 in a liquid oxygen bath a (evaporator). The oxygen vapours condense first of all and partly run into the column 9 and partly collect on a bottom 1). The oxygen vapours in the column are washed ,in an increasingly thorough manner by the liquid oxygen mixture passing through the same until pure nitrogen is present in the tubes 0 of the condenser The said nitrogen, insofar -as it is not required as washing liquid in the column, is collected inliquid form on the bottom 2. From here the liquid nitrogen is conveyed through a pipe 1' into the lower part 9 of a coil 8, to which liquid oxygen is conveyed from the bath n by the pipeg-f and from which it is removed through the pipe h after evaporating during its passage ing, subject to pressure, through the centre passage 22 in the bottom p passes over the exterior of the coil 8 and is thus condensed by the cold liberated therein by the evaporation The gaseous nitrogen fiowof the oxygen. In this manner, as stated above, the quantity of washing nitrogen i the upper column is added to.

. In the device illustrated in Fig. 2 the liquid oxygen in the evaporator n is at a temperature of 182 C. At this temperature the liquid oxygen passes into the warmer part of the lower stage wherein, at the upper end, the temperature is that'of pure nitrogen at 5 atmospheres, namely-178. Since, therefore, the temperature in the uppermost zone of the lower pressure stage is -178 and the nitrogen vapours rising under pressurefrom the column g are, above the reducing valve e, at a temperature of about 175, there is consequently a temperature drop of 7. To utilize this fully the bottom of the evaporator is made deeper or runs to a point towards the middle and the coil 8, to which the liquid oxygen is conveyed from the evaporator by way of the pipe f, is extended into the vicinity'of the pressure reducing valve 6, so that the outlet end h thereof is disposed in the zone at which the temperature is 175. The liquid oxygen comin from a ,zone with a temperature of 182 1s continuously heated, evaporates ever more rapidly and thus 'gives up anincreasing amount of cold, resulting in an increasing condensation of the oxygen contained in the rising vapours rich in nitrogen, said condensation being carried so far that pure nitrogen vapours are present at the upper end of thecoil. Complete condensation of the oxygen contained in the rising nitrogen vapours is achieved since the quantity of oxygen evaporating in the coil 8 is at least twice as great as the quantity of oxygen in the rising nitrogen vapours.

Due to the complete condensation of the oxygen a return fiow of liquid nitrogen, to produce a rectification efiect, through the column from the condenser tubes 0, as in. the first described method of operation, is not necessary. On the contrary the total quantity of nitrogen liquefied by'the condensation effect-is at disposal as washing nitrogen in the low pressure part of the apparatus.

The coil .5- therefore partlyreplaces the rectification column otherwise usual in the pressure stage, the resultbeing that considerably more washing nitrogenis at disposal in the upper column than when the lower stage is constituted by a pure rectification column.

What I claim is 1. The process of separating air into oxygen and nitrogen, which includes liquefying cold compressed air at-the lower part of one column by indirect contact with liquid rich in oxygen, delivering said liquid air to the column, rectifying the last mentioned liquid by descent in the column from the point of admission to the place of indirect contact with the air to be liquefied, deliverin the vapors rising from said point 'of liqui ad- IIllSSlOIl to two cooling zones in series to completely liquefy said vapors-in two sta es, re-

- turning the liquid from the first stage 1n contact with and countercurrent to the vapors rising from said'point of'liquid admission, withdrawing the liquid formed in said second zone, withdrawing li uid from the bottom of said column, and elivering both of said last mentioned liquids to a second column under lower pressure and at difierent elevations therein, vaporizing a portion of the liqond and lower pressure column at difierent elevations therein the liquid formed in said second stage and the liquid collecting in the bottom of said first mentioned column, the

liquefaction of the vapors in the second stage ends of the latter the liquid condensed at still higherpressure by the liquid collecting in the bottom of said column, completely liquefying the vapors in the higher pressure column above the point of admission of said liquid, and in two successive stages, returning the liquid formed in the first stage to said column, delivering to the lower pressure column the liquid formed in the said second stage and the liquid collecting in the bottom of'the higher pressure column, efiecting the said liquefaction in the said two stages by the liquid collecting in the bottom of the lower pressure column, andfwithdrawingas entirely separate gases the separate vapors produced from the liquids used in cooling said first and second stages;

In testimony whereof I hereunto aflix my signature. I

. ADOLF MESSER.

uid in the lower end of the second column by the liquefaction in the aforementioned second stage, withdrawing a further portion of said last mentioned liquid and vaporizin it to efli'ect the cooling in the aforementione first stage, withdrawing all of the vapor produced in said cooling of said first stage, and

separately withdrawing all of the gas pro duced in said cooling of said second stage.

2. The pr'ocess of separating air into oxy-' gen and nitrogen, which includes-maintaining two rectification columns under difierent pressures, delivering to the first and higher pressure column at a point intermediate the ends of the latter theliquid air which has been liquefied in the lower portion of said 1 column through indirect contact with the liquid collecting in said lower portion, completely liquefying the vaporsfrising in the upper portion of said last mentioned column and in two successive stages, the li uid formed in the first stagebelng permltte to descend in said colummdeli'v'ering to the sec- 

